1. Field of the Invention
The present invention relates to recessed light fixtures, more specifically to LEDs used in recessed down light fixtures.
2. Description of Related Art
Recessed light fixtures are known, and are typically used when it is desirable to minimize the projection of the light fixture below the ceiling surface. Recessed light fixtures, as opposed to light fixtures that substantially extend below the ceiling surface, tend to be more aesthetically appealing and provide a cleaner look when installed. Thus, recessed light fixtures tend to be used in commercial settings such as offices and the like.
Light emitting diodes (LEDs) have been used since the early 1970s as a reliable low energy light source for indicator lights. LEDs are generally single frequency light sources, but in the early 1990s, blue LEDs were introduced, which made it possible to generate white light by coating the blue LED die with phosphor. The first white LEDs had a high color temperature of the order of 5000° K to 6000° K and were low in power. The most common type is the familiar 5 mm LED. Around the year 2000, higher power LEDs from 1 to 5 watts became available and lower color temperature range increased to as low as 2700° K. Color rendering was low and efficacy was 30 to 50 Lumens per watt.
Today, LEDs are available for a wide range of applications. The 5 mm LED is still the most commonly used for indicators, and sometimes for illumination. Single die power LEDs produce in excess of 5 watts of power at an efficacy of 50 to 100 lumens/W, depending on color temperature and CRI (Color Rendering Index). As a general rule, the lumen output drops as the color temperature is reduced and CRI increased. Both CRI and color temperature are functions of the phosphor coatings that are applied to the blue LED die.
Power LEDs are also constructed from multiple lower power dies that are wired on the same substrate. Multi-die LEDs may offer a higher efficacy than single die LEDs. These lower power dies, of the order of 70 to 100 mW, are very efficient when powered one at a time, and may be packaged individually as a Miniature Power LED.
General illumination has been driven primarily by incandescent bulbs and gas discharge tubes, including fluorescent, and Ceramic Metal Halides. Gas discharge tubes were introduced as a low energy light source to replace incandescent. They also offered a longer operating life. Had it not been for their higher CRI, simple design and lower cost, incandescent lamps would have long been extinct.
LEDs are available today in color temperatures ranging from 6000° K to 2700° K, and as high as 98 CRI. Day light and low voltage halogen incandescent lamps have a CRI of 100. As mentioned earlier, the LED lumen output drops as the CRI of a white light LED is increased. This puts a limit on the maximum CRI of an LED for an optimum lumens-cost-performance. CRIs of 60 to 75 are the most common and are used in street illumination and car headlights, where color rendering is not critical. A CRI less than 85 is used in general illumination since it is compatible with the CRI of the majority of fluorescent lamps. LED lamps with a CRI of 90 or higher are considered ideal replacements of quality incandescent lamps, such as low voltage Halogen.
The efficacy of a typical power LED can reach over 100 lumens/W, which makes it a feasible replacement of both incandescent and fluorescent lamps. Like fluorescent tubes, LEDs are current driven devices, except that an LED is driven by a DC current at a lower voltage. If the light output of an LED is required to be constant against input power changes, the drive current should be regulated, otherwise, the LED drive DC current may be allowed to vary, provided it does not exceed the maximum rating.
LED drivers may require isolation from AC line voltage for safety, especially if the LEDs are accessible. If the LEDs are encased in an approved dielectric barrier, electrical isolation will not be required, and LEDs may be driven directly from AC sources.
While incandescent lamps rely on heat and high temperature to produce light, LEDs produce light from changes in quantum energy levels of electrons in the LED semiconductor chip. However, LEDs, like all electronic devices, are not without losses, where heat is generated as a by-product, and needs to be dissipated before it causes excessive rise in LED junction temperature.
Dissipating LED heat becomes more critical as the LED is driven closer to its maximum rating. This is usually the case when maximum light output is required by design, which comes with the penalty of lower life and higher losses.
Critical heat dissipation can be avoided by reducing the amount of heat generated. This is accomplished by reducing the light output of the LED, which also improves the efficacy and increases the life of the LED.
The manufacturing process of white light LEDs yields a wide variance in voltage, luminosity, and color temperature. Each packaged LED goes through a series of tests and is ranked in “Bins” according to performance. There are three main bins commonly used today: voltage, lumen output, and color temperature. Of the three, color temperature is the most critical, since it is readily detected by the human eye. Each color temperature bin is approximately 200° K apart from an adjacent bin on the chromaticity chart.